This invention relates to endoluminal stents and techniques for their manufacture.
Recent years have witnessed increased development and use of stents, particularly those adapted for use in body lumens such as blood vessels, urological passages and biliary passageways. Stents have found increasing use as an adjunct to angioplasty, a technique in which an obstructed blood vessel is dilated, as by use of a lob balloon dilatation catheter. Although angioplasty has proved to be highly effective in the treatment of arterial obstructions, a substantial number of cases result in restenosis, that is, a tendency for the obstruction to reestablish itself. By placing a stent within the dilated region of the blood vessel, the prospects for avoiding post-procedure luminal obstruction are considered to be substantially improved.
Stents typically may be considered as defining a generally tubular structure that is adapted to be maintained in a small diameter (low profile) and expanded to a larger diameter. The low profile facilitates its advancement through and placement at the desired site in the blood vessel where it then can be expanded to the larger diameter, configuration in which it is in firm engagement with the inner lumen of the vessel and can serve as a scaffold to maintain that portion of the vessel open. Among the different types of stents are those that are self-expanding under their inherent resilience, those that are expandable by a trigger mechanism, such as by making the stent from a shape memory alloy and then heat activating the alloy to its expanded configuration, and balloon-expandable stents in which the stent is expanded by an inflatable balloon from its low profile to its expanded, deployed configuration. Regardless of the type of stent, its delivery device typically is removed after the stent has been expanded and deployed.
In many applications that lend themselves to use of a stent, the body lumen is quite small and the stent similarly must be correspondingly small. For example, stent adapted for placement in a coronary artery may have an initial low profile diameter as small as 1.5 millimeter and may be expanded to a diameter of up to about 3.5 millimeter or more. The stent, which may be formed from an arrangement of wires, cut or etched tubes, or other configurations may be considered as having a wall thickness of the order of 1 millimeter or less. Numerous structural configurations may be employed including various arrangements of struts, zigzag members and flat sheets rolled into tubes, among others. Among the important objectives in the design and manufacture of a stent is that the stent should be capable of assembly and fabrication with a minimum of difficulty. That objective can be challenging particularly with small stents in which it may be difficult to assemble or align the individual components.
For those stents that are formed from a continuous, unitary material, such as a metal tube or sheet, the stent pattern may be formed by a variety of techniques, including the use of laser cutting and photochemical etching, among others. The prior art describes techniques for forming a balloon expandable metal stent from a tube by cutting slits in the tube (e.g., Palmaz U.S. Pat. No. 4,773,655), by laser cutting defined patterns in such a tube (e.g., International application PCT/US96/02615) or by photochemically etching (e.g., Lau, U.S. Pat. No. 5,421,955). U.S. Pat. No. 4,680,031 discloses a tubular stent adapted for use with a heart valve in which the stent is formed from a flat metallic sheet in a predetermined pattern, such as by chemical etching, with the sheet then being rolled into a tubular shape and its edges secured to each other as by welding.
The fabrication of the stent pattern in a flat sheet and then rolling the patterned sheet into a tubular stent form may have advantages over forming the stent pattern directly in a tubular member. Some difficulty, however, may be encountered when forming the patterned flat sheet into a tubular configuration and then securing the longitudinal edges of the sheet together to form the tube. With a tube so made, the longitudinal margins or edges must be properly aligned and effectively secured. In many cases, it may be desirable that the wall thickness of the stent be uniform and not substantially greater than that of the sheet from which the tube is made. That can present assembly difficulties, particularly for smaller dimensioned stents.
Among the advantages of fabricating the stent in a photochemical etching procedure is that intricate patterns, essentially of any configuration, can be formed with a high degree of precision. The precision may be considered to be greater than that achievable with direct laser cutting alone because of practical limitations on the diameter of the laser beam necessary to have sufficient power to effect the cutting. A hybrid technique is described in U.S. Pat. No. 5,421,955 in which a tube is coated exteriorly with an etchant-resistant coating that is selectively removed in a desired pattern by a laser beam. The tube then is filled with wax and is exposed exteriorly to the etchant. Because the etchant is applied only to the outside of the tube, there may be some uncertainty as to the internal configuration and dimension of the pattern on the inner surface of the tube. That may result in a lack of uniformity and predictability in the function and operation of the stent. In contrast, when forming a stent from a flat sheet, the identical pattern can be provided, in registry, on both sides of the sheet and the etching can take place simultaneously from both sides of the sheet, thereby assuring that the pattern and dimensions of the pattern on the inner and the outer surfaces of the ultimately formed stent will be essentially identical.
It would be desirable to provide a stent construction and technique for facilitating assembly of tubular stents from a flat sheet of material and in which the alignment, registration and connection between the longitudinal margins of the stent is true, secure and is effected in a simplified technique.
In practicing the invention, a tubular stent is formed from a sheet of material that has been coated, on both sides, with an etchant-resistant coating that defines the pattern of the stent to be formed. The sheet then is exposed to a chemical etchant to remove the exposed material. The etchant-resistant coating then is removed and the sheet is rolled into a cylinder in which the longitudinal edges of the rolled sheet are aligned and secured. In accordance with the invention, the longitudinal marginal edges are configured to engage each other easily and can be secured while so engaged. The marginal edges may be configured to interlock in a manner that will assure precise registration of the edges and the formation of a true cylindrical stent. The engaging portions of the margins along the longitudinal edges of the stent may be formed with various patterns to facilitate engagement and proper registration of the margins.
It is among the objects of the invention to provide a stent and technique for its manufacture that can be formed using conventional photochemical etching techniques that can be accurately assembled from a flat sheet into a cylindrical configuration; that can be rolled into a cylindrical configuration with its longitudinal margins being securely and accurately engaged; and can be made in any pattern, including multiple connected modules.